The invention relates to the general field of giant magnetoresistive read heads for disk systems with particular reference to improving the AMR/GMR ratio.
Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG. 1. The magnetic field that xe2x80x98writesxe2x80x99 a bit at the surface of recording medium 15 is generated by a flat coil, two of whose windings 14 can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces 12 and 13 which, while being connected at a point beyond the top edge of the figure, are separated by small gap 16. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium 15.
The present invention is directed towards the design of read element 20 which can be seen to be a thin slice of material located between magnetic shields 11 and 12 (12 doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor 20 is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as an increase in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said increase being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance. We refer to the maximum increase in resistivity due to this anisotropy as AMR (anisotropic magneto-resistance).
It is now known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 2. In addition to a seed layer 22 on a substrate 21 and a topmost cap layer 27, these key elements are two magnetic layers 23 and 25, separated by a non-magnetic layer 24. The thickness of layer 24 is chosen so that layers 23 and 25 are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other""s magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers 23 and 25 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 28 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 23 to 25 (or vice versa). However, once these electron xe2x80x98switch sidesxe2x80x99, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one of the layers 23 and 25 is permanently fixed, or pinned. In FIG. 2 it is layer 25 that is pinned. Pinning is achieved by first magnetizing the layer (most often by depositing it in the presence of a magnetic field) and then permanently maintaining the magnetization by overcoating with a layer of antiferromagnetic material, or AFM, (layer 26 in the figure). Layer 23, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface 15 of a magnetic disk).
The structure shown in FIG. 2 is referred to as a top spin valve because the pinned layer is at the top. It is also possible to form a xe2x80x98bottom spin valvexe2x80x99 structure where the pinned layer is deposited first (immediately after the seed and pinning layers). In that case the cap layer would, of course, be over the free layer.
Ideally, while the device is being operated, free layer 23 would be given a bias whereby its direction of magnetization was mid-way between that of the pinned layer and a direction orthogonal to that. Thus, the GMR effect would be present, but not to its full extent. Then when the magnetization direction of layer 23 was changed by the field associated with a bit on the surface of 15, the resistance of the device would either increase or decrease depending on the direction of the field coming from surface 15. In practice other requirements (such as good signal linearity and good cross-track asymmetry) make it necessary for the bias to be close to zero. Under these conditions, the AMR effect from the free layer degrades the peak-to-peak signal asymmetry. Additionally, a high AMR/GMR ratio causes a larger signal asymmetry variation.
The relationship between the resistance R of a spin valve structure and the angle between the magnetization directions xcex8f and xcex8p of the free and pinned layers, respectively is given by:
R=Rs[1+0.5xc3x97GMRxc3x97{1xe2x88x92cos(xcex8f xe2x88x92xcex8p)}+AMRxc3x97{cos xcex8f}2]xe2x80x83xe2x80x83(1)
where Rs is the saturation (minimum achievable) resistance.
Equation (1) confirms that the signal contribution from AMR strongly depends on the AMR/GMR ratio. Reducing this ratio is not, in general, a straightforward thing to do since most steps that can be taken to reduce the AMR (such as choice of seed layer or a thinner free layer) also result in a reduction of the GMR. The present invention describes a spin valve structure in which both the AMR and the AMR/GMR ratio have been reduced.
A routine search for prior art was performed. While no references to the exact structure taught by the present invention were found, several references of interest were encountered. For example, Gill (U.S. Pat. No. 5,898,549) forms a pinned layer from three separate pinned layers. The first of these is formed on the pinning layer and, together with the second pinned layer is formed of a high resistivity material such as NiFeCr. They are separated by an anti-parallel coupling layer. The third pinned layer is of low resistivity material such as cobalt.
In U.S. Pat. No. 5,920,446, Gill describes a laminated free layer formed from two ferromagnetic layers separated by a non-magnetic, conducting spacer layer. A key feature is that the two outer layers of the laminate are coupled in an anti-parallel configuration. This arrangement allows the device to operate without a pinned (or pinning) layer.
Mao et al. (U.S. Pat. No. 5,764,056) teaches use of Nickel-Manganese as a pinning layer for, a pinned layer that is a laminate of two ferromagnetic materials separated by a non-magnetic conducting layer.
It has been an object of the present invention to provide a spin valve structure having both a low AMR as well as a low AMR/GMR ratio.
Another object of the invention has been that said spin valve structure be compatible with existing techniques for manufacturing spin valves.
A further object of the invention has been to improve cross-track asymmetry and signal linearity in magnetic read heads.
These objects have been achieved by modifying the free layer whereby an extra layer of a highly resistive or insulating material is inserted through the middle of the free layer. This layer is from 3 to 15 angstroms thick and serves to reduce the Anisotropic Magneto-resistance contribution to the total magneto-resistance of the device. This reduces the GMR contribution only slightly but cuts the AMR/GMR ratio in half, thereby improving cross-track asymmetry and signal linearity.